Abstract
High proton selectivity is the ultimate aim for the ion exchange membranes (IEMs). In this study, two kinds of sulfonated polyimides (SPI)—non-fluorinated and fluorine-containing polyimide—with about 40% sulfonation degree were synthesized by one-step high temperature polymerization. High proton selectivity IEMs were prepared and applied in vanadium flow batteries (VFB). The chemical structures, physicochemical properties and single cell performance of these membranes were characterized. The results indicate that high molecular weight of SPIs can guarantee the simultaneous achievement of good mechanical and oxidative stability for IEMs. Meanwhile, the proton selectivity of SPI membrane is five times higher than that of Nafion115 membranes due to the introduction of fluorocarbon groups. Consequently, the single cell assembled with SPI membranes exhibits excellent energy efficiency up to 84.8% at a current density of 100 mA·cm−2, which is 4.6% higher than Nafion115. In addition, the capacity retention is great after 500 charge–discharge cycles. All results demonstrate that fluorinated SPI ion exchange membrane has a bright prospect in new energy field.
Highlights
New energy sources, such as solar, wind, geothermal, and hydro energy have gained wide attention during the past decades because they are renewable and eco-friendly [1,2,3].the discontinuous, unstable, and uncontrollable output of those energies limits their practical application to a large extent
The results indicated that branched sulfonated polyimide (bSPI) showed better thermal and mechanical properties than linear sulfonated polyimides (SPI); proton conductivity and vanadium ion permeability both increased with the development of degree of branching (DB)
SPI was prepared by a typical one-step high temperature polymerization method, and the corresponding chemical reaction equations were shown in Scheme 1
Summary
New energy sources, such as solar, wind, geothermal, and hydro energy have gained wide attention during the past decades because they are renewable and eco-friendly [1,2,3]. Proton conductivity of SPI membranes increased from 0.012 to 0.051 S·cm−1 for the sulfonation degree ranged from 40% to 89%, and the permeability of vanadium ion was much lower than Nafion117. 30 mA·cm−2 , and SPI(BAPP) which use 2,2-bis[4-(4-aminophenoxy)phenyl] propane as diamine monomer has the best VFB performance, the energy efficiency of SPI(BAPP) was 78% at the current density of 30 mA·cm−2 , about 13% higher than that of Nafion117 They synthesized a kind of branched sulfonated polyimide (bSPI) to prepare membranes for VFB application, and the degree of branching (DB) of membranes was investigated in detail. In order to pursue the appropriate polymer materials that have good mechanical properties and high proton conductivity for VFB, two kinds of SPI with almost the same degree of sulfonation were used as IEMs for VFB applications. It is important that the single cell performance, open circuit voltage, and capacity retention were tested for 500 charge–discharge cycles to further verify the service life of SPI ion exchange membranes
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